1. Field of the Invention
The present invention relates to a laser beam scanning apparatus that scans a bar code with a laser beam and detects the laser beam reflected from the bar code.
2. Description of the Related Art
Recently, POS (Point-Of-Sales) systems have rapidly spread, which enable goods to be checked out by reading bar code information attached to them.
Since the POS systems make it possible to check out goods simply by facing them toward a read unit, the load on the operator has been reduced. As a laser beam scanning apparatus for reading bar code information in the check-out operation, a stationary scanner is mainly used.
The POS systems and other similar systems are demanded to scan a bar code with laser beams in a plurality of different directions so that goods information can be read from the bar code irrespective of the posture of the bar code in which it is faced toward the read unit.
Accordingly, the conventional practice is such that a laser beam emitted from a laser light source is reflected by a rotating polygon mirror, and a plurality of mirrors are disposed on the optical path of the reflected laser beam at different angles of inclination, thereby splitting a single laser beam into a plurality of scanning beams and directing them toward the read unit in a plurality of different directions.
However, since the volumetric capacity of the apparatus becomes considerably large depending on the number of mirrors used and the arrangement thereof, a holographic diffraction plate, which changes the direction of a laser beam by the action of diffraction, is disposed on a scanning optical path in order to minimize the overall size of the apparatus.
It should be noted that a part of the laser beam reflected from the bar code returns to the polygon mirror along the same optical path as that of the emergent beam and is then focused on a laser beam detector by a condenser lens. The laser beam detector photoelectrically converts the laser beam and outputs an electric signal. Bar code information is read from the differential waveform of the electric signal.
FIG. 15 shows an output waveform of the laser beam detector in the above-described laser beam scanning apparatus, together with the corresponding differential waveform.
As shown in the figure, the signal output (voltage) of the laser beam detector is larger during the laser beam scanning of a white bar of the bar code than during the scanning of a black bar.
In signal processing, a slice ratio for an upper limit and a limiter level for a lower limit are set. It should be noted that the limiter level is set for the purpose of eliminating the noise component. Accordingly, a component lower than the set level is neglected in the following processing.
The interval from the time the differential wave exceeds the positive limiter level until it subsequently exceeds the negative limiter level (i.e., between (a) and (b) in FIG. 15) is recognized as a black bar. The interval from the time the differential wave exceeds the negative limiter level until it subsequently exceeds the positive limiter level (i.e., between (b) and (c) in FIG. 15) is recognized as a white bar.
The course of the laser beam incident on the laser beam detector varies according to the direction of the bar code facing toward the read position: In one case, the laser beam passes through the holographic diffraction plate; in another case, it does not. In a case where the laser beam passes through the holographic diffraction plate, it passes through the plate twice, that is, when the laser beam is going to scan the bar code, and after it has been reflected from the bar code.
Since the diffraction efficiency of the holographic diffraction plate is not 100%, when the laser beam passes through it, the intensity thereof attenuates. Accordingly, when the diffraction efficiency of the holographic diffraction plate is low, the intensity of the laser beam incident on the laser beam detector becomes extremely lower than in a case where the laser beam does not pass through the holographic diffraction plate.
Consequently, if the limiter level (limiter level 1) is set relatively high, as shown in FIG. 15, all the signal information carried by the laser beam passing through the holographic diffraction plate may be cut off.
If the limiter level is set as low as the limiter level 2 in order to prevent the occurrence of the above-described problem, the noise level of the laser beam when it does not pass through the holographic diffraction plate, in which case the detected laser beam intensity is relatively high, may exceed the limiter level and be erroneously recognized as bar code information.
Thus, the margin allowed for a limiter level to be set and the margin allowed for the diffraction efficiency of the holographic diffraction plate are extremely small. Accordingly, it is difficult to arrange the apparatus so that information can be read from bar codes even more accurately.